We have studied 3 cases of leukemias with mixed phenotype and 2 lymphoblastic lymphomas trying to further characterize them. These cells respond to treatment with IL-2 as do normal LAK cells, in that they develop the ability to kill tumor cells. Their phenotype changes to resemble that of the mature LAK cell. Using these cases, we propose that the normal LAK progenitor cell is a T/myeloid stem cell which can be differentiated by IL-2 to become a mature LAK cell, while losing its myeloid characteristics.